Showing papers in "Materials evaluation in 1994"

Lamb wave reflection for the quick nondestructive evaluation of large composite laminates

Abstract: The possibility of using Lamb waves for the long range nondestructive evaluation (NDE) of composite laminates has been investigated. Three different possible testing strategies using the S[sub 0] Lamb mode--S[sub 0] mode transmission, S[sub 0] mode conversion to A[sub 0], and S[sub 0] mode reflection--are described and evaluated. It is shown that the S[sub 0] mode reflection technique is the most promising method for long range inspection, 10--20 mm diameter delamination being detectable at a range of at least 500 mm. The time taken to test along a line using this technique is similar to that required to test at a point in conventional ultrasonic inspection, so considerable savings in testing time may be obtained.

Ultrasonic guided wave inspection concepts for steam generator tubing

Abstract: Some very exciting and promising results have been obtained with respect to the utilization and guided wave techniques for inspecting steam generator tubing. In addition to some theoretical considerations that were studied recently, work has been carried out in special probe design and development. This special probe is used in demonstration of law detection feasibility, and in understanding the conceptual development of a complete flaw detection system. This includes transducer, pulser-receiver system, and appropriate signal processing and pattern recognition software for reliable inspection. Ultrasonic NDE techniques have progressed quite rapidly during this decade for two principal reasons: advanced signal processing, and the use and understanding of multi-mode ultrasonic wave propagation. Both concepts are useful in the proposed work on guided wave propagation in steam generator tubing. These new directions go beyond the use of normal beam longitudinal waves and angle beam shear waves for inspection. Guided waves such as surface and Lamb waves can be used to monitor larger volumes of material with greater efficiency. The generation of these waves, however, is more complex. Theoretically one can produce a large number of modes in a structure with a simple loading arrangement. However, the generation of sufficient amounts of energy inmore » a specific mode strongly depends on several factors. They include the loading system, angle of attack, probe frequency, frequency bandwidth, and a whole host of special transducer design and instrumentation parameters.« less

Self-Nulling Eddy Current Probe for Surface and Subsurface Flaw Detection

Abstract: An eddy current probe which provides a null-signal in the presence of unflawed material without the need for any balancing circuitry has been developed at NASA Langley Research Center. Such a unique capability of the probe reduces set-up time, eliminates tester configuration errors, and decreases instrumentation requirements. The probe is highly sensitive to surface breaking fatigue cracks, and shows excellent resolution for the measurement of material thickness, including material loss due to corrosion damage. The presence of flaws in the material under test causes an increase in the extremely stable and reproducible output voltage of the probe. The design of the probe and some examples illustrating its flaw detection capabilities are presented.

Impulse radar reflection waveforms of simulated reinforced concrete bridge decks

Abstract: This paper presents the findings of an experimental study on the use of ground penetrating radar (GPR) for nondestructive evaluation of reinforced concrete bridge descks. An impluse radar PS-24 system was used for this study. Fifteen concrete bridge deck specimens were cast in three batches. The internal conditions were varied as follows : with steel reinforcement and unreinforced, with air cracks, with plain water-filled cracks, and with saline water-filled cracks.

A novel microwave method for surface crack detection using higher order waveguide modes

Abstract: A novel microwave nondestructive testing method for fatigue/surface crack detection on metal surfaces is described. This technique uses an open-ended waveguide to probe the surface of a metal. In the absence of a crack, the metal surface is seen as a relatively good short circuit load that will not generate any higher order modes. However, in the presence of a crack higher order modes are generated, and subsequently the transverse electric field components of these modes are used as a qualitative indication of the existence of a crack. The experimental and theoretical foundation of this technique is given, along with several examples. It is shown that fatigue/surface cracks with widths less than 5 mm can be detected at 38 GHz. The optimum measurement parameters are theoretically predicted and confirmed by experiments.

Millimeter-wave imaging for nondestructive evaluation of materials

Abstract: A millimeter-wave imaging system has been developed in the W band (75--110 GHz) for nondestructive evaluation of low-loss materials. The system employs a focused beam to provide spatial resolution of about one wavelength. A plane-wave model is used to calculate the effective reflection (or transmission) coefficient of a multilayer geometry. Theoretical analysis is used to optimize the measurement frequency for higher image contrast and to interpret the experimental results. Both reflection and transmission images, based on backscattered and forward-scattered powers, were made with Kevlar/epoxy samples containing artificially introduced defects such as subsurface voids and disbonds. The results indicate that millimeter wave imaging has high potential for noncontact detection of defects in low-loss materials.

Acoustic emission from single point machining: Source mechanisms and signal changes with tool wear

Abstract: Acoustic emission (AE) was monitored during single point, continuous machining of 4340 steel and Ti-6Al-4V as a function of heat treatment. Heat treatments that increase the strength of 4340 steel substantially increase the amount of AE produced during deformation, while heat treatments that increase the strength of Ti-6Al-4V dramatically decrease the amount of AE produced during deformation. There was little change in root-mean-square (rms) AE level during machining for either alloy as a function of prior heat treatment, demonstrating that chip deformation is not a major source of AE in single point machining. Additional data from a variety of materials suggest that sliding friction between the nose and/or flank of the tool and the newly machined surface is the primary source of AE. Changes in AE signal characteristics with tool wear were also monitored during single point machining. No signal characteristic changed in the same way with tool wear for all materials tested. A single change in a particular AE signal characteristic with tool wear valid for all materials probably does not exist. Nevertheless, changes in various signal characteristics with wear for a given material may be sufficient to be used to monitor tool wear.

Neural network detection of fatigue crack growth in riveted joints using acoustic emission

Abstract: Author Adriano F de Almeida Title Neural Network Detection of Fatigue Crack Growth in Riveted Joints Using Acoustic Emission Institution Embry-Riddle Aeronautical University Degree Master of Science in Aerospace Engineering Year 1994 The purpose of this research was to demonstrate the capability of neural networks to discriminate between individual acoustic emission (AE) signals originating from crack growth and rivet rubbing (fretting) in aluminum lap joints AE waveforms were recorded during tensile fatigue cycling of six notched and riveted 7075-T6 specimens using a broadband piezoelectric transducer and a computer interfaced oscilloscope The source of 1,311 signals was identified based on triggering logic, amplitude relationships, and time of arrival data collected from the broad-band transducer and three additional 300 Hz resonant transducers bonded to the specimens The power spectrum of each waveform was calculated and normalized to correct for variable specimen geometry and wave propagation effects In order to determine the variation between individual signals of the same class, the normalized spectra were clustered onto a two-dimensional feature space using a Kohonen self organizing map (SOM) Then 132 crack growth and 137 rivet rubbing spectra were used to train a back-propagation neural network to provide automatic pattern classification Although there was some overlap between the clusters mapped in the Kohonen feature space, the trained back-propagation neural network was able to classify the remaining 463 crack growth signals with a 94% accuracy and the 367 rivet rubbing signals with a 99% accuracy

Evaluation of NDE techniques for detecting grout defects in cable stays

Abstract: Problems with corrosion of steel strands in grouted bridge cable stays have raised concern about the integrity of the grout layer protecting the strands. Bleed water that may accumulate in the grout layer if voids or bubbles are trapped during the grouting process can lead to corrosion of the cables. This research study was undertaken to evaluate various nondestructive evaluation (NDE) methods for inspection of this grout layer. Methods which were studied include ultrasonics, film radiography, computed tomography (CT), and neutron radiography. The feasibility of using computed tomography for inspection of the grout layer has been demonstrated. Voids as small as 0.04 in. (1 mm) in diameter were identified in both the annular region of grout and inside the steel cable bundle. Although some of these voids were manufactured, many of those found were a result of the grouting operation during fabrication of the specimens, an observation which lends urgency to the need for an inspection system. Laboratory experiments were conducted with a prototype ultrasonic inspection unit which utilizes two rolling contact probes with 500 kHz transducers. A digital signal subtraction technique was found to be particularly useful in analyzing the received signals. The ultrasonic system was successful in identifying the location of defects, but information regarding the size and content of the defects was inconclusive. Three distinct regions exist along the cable which will most likely require different inspection schemes. It is envisioned that the main section of cable will be inspected by one technique, possibly ultrasonics, while the lower regions of cable, where extra layers are added for fire and crash protection, will require a radiographic technique. The third region, which includes the saddles and anchorages, will likely require a third inspection technique, such as a portable linear accelerator.

Three-dimensional ultrasonic imaging of defects and damage in composite materials

Abstract: Based on the analysis of ultrasonic A-scans, a scanning system was developed to construct a pseudo three-dimensional image of discontinuity and damage in composite materials. Special echo detection techniques were successfully developed and applied for detecting delaminations due to low velocity impact on a thin composite plate and planar inclusions in thick plates. The same imaging procedure was applied in both cases. Full volume evaluation of a defect/damage map was practically possible by developing an archival storage database system built with an efficient compression scheme and a pattern matching algorithm that is best suited for the particular scanning system.

Permittivity of Portland cement concrete at low RF frequencies

Abstract: Portland cement concrete (PCC) specimens were cast and most cured for 28 days. Their complex permittivity was measure dover a frequency ranger of 100 kHz--40 MHz as part of an ongoing study intended to develop a database of electromagnetic properties of PCC. Such information would allow the improvement of current nondestructive evaluation (NDE) methods used to evaluate concrete structures. Low RF (radio frequency) measurements were carried out using a parallel plate capacitor setup connected to an HP 4195A Network/Spectrum Analyzer, Using a complex capacitance model for the capacitor, the complex permittivity of twelve different PCC mixes was evaluated; measurements were performed on four specimens per batch. The selected mixes included three water-to-cement (w/c) ratios, two aggregate types, and two levels of entrained air content; Type 1 portland cement was used. Preliminary measurements showed that the dielectric constant of the concrete specimens varied over frequency from nearly 150 at 1 MHz to around 6 at 40 MHz after 28 days of moist curing. Measured loss tangent was in the range of 0.86--1.8 at 1 MHz and 1.0--2.9 at 40 MHz. A significant effect of w/c ratio, type of aggregate, air entrainment, and the curing state of specimens was observed.

Statistical analysis of UT corrosion data from floor plates of a crude oil aboveground storage tank

Abstract: Nondestructive data collected during inspection for slow corrosion attack on large structures such as offshore oil platforms, oil pipelines, or aboveground storage tanks (ASTs), pose a dilemma. On the one hand, inspecting the entire large structure produces such a vast amount of data that it is unmanageable. On the other hand, it is difficult to approximate how much sample data would be sufficient to estimate the corroded state of the structure. The extreme value statistical analysis of the selected areas of a large structure offers a fair estimate of the maximum corrosion pit depth without examining the total area of a large structure. This statistical method was used successfully on the corrosion data gathered by ultrasonic thickness gaging a floor plate of a crude oil AST.

Laser ultrasound directivity in CS-3 ceramic

Abstract: A single optical fiber was used to deliver laser energy for the generation of ultrasound in CS-3 type ceramic blocks. The attenuation coefficient of light in the ceramic was determined. The experimental and analytical directivity patterns were also obtained for longitudinal and shear waves. The theoretical predictions did not agree well with measured data for longitudinal waves. Potential causes for this disagreement are discussed.

Trends in radiologic NDT

Abstract: In this article, the author tries to look ahead to see what is coming in the field of nondestructive testing (NDT) using radiation methods. Radiological NDT has changed since gamma ray and x-ray inspection came into widespread use more than 50 years ago. Even the name has changed. Instead of referring to most radiation inspection approaches as radiography as one once did, the ''umbrella'' term is now radiology, as defined by ASTM, ''the science and application of x-rays, gamma rays, neutrons and other penetrating radiations.'' Radiography refers to film or film-like methods. Radioscopy refers to electronic methods ''that follow very closely the changes with time of the object being imaged.'' In addition, radiology includes tomography, backscatter, gaging, and a host of other radiation inspection methods. All of these techniques are likely to change as we move into the 21st century. The author's favorites as radiologic methods and applications that will see much greater use in this decade and the early part of the twenty-first century can be summarized as follows: (1) x-radioscopic digital systems for manufacturing cations and in-service inspections and process control applications; (2) computerized radiological systems, radiographic, radioscopic, tomographic, and laminographic with capability for data exchange, use withmore » NDT workstations and fully automated systems; (3) small inspection volume, low-cost CT systems; (4) x-ray image detection systems based on microfocus/image magnification; (5) small format, high sensitivity x-ray cameras; (6) large area photoconductive x-ray cameras; and (7) high output and transportable radiation sources.« less

Video formats and resolution

Abstract: Closed-circuit television (CCTV) systems including lenses, cameras, monitors and recording devices (see figure 1) are being used ever increasingly in Nondestructive Testing (NDT) today. CCTV is similar to Broadcast video except that its transmitted signal is contained within the system and therefore allowed by the Federal Communications Commission (FCC) to have a greater bandwidth (frequency range) and the potential for more resolution. Applications for CCTV range from simple black and white (B&W) cameras that monitor the remote position of an ultrasonic transducer to expensive, radiation-resistant color cameras for critical nuclear fuel bundle inspections.

Acoustic surface waveguides for acoustic emission monitoring of fiber-reinforced plastic structures

Abstract: Acoustic surface waveguides are developed to enhance the transmission of acoustic emission (AE) signals in high attenuating fiber-reinforced plastic (FRP) structures. In this paper, the design of the surface waveguide system and the source location technique are described. Experimental results of using a surface waveguide for AE monitoring of a FRP composite pressure pipe are presented to demonstrate the effectiveness of the proposed waveguide system. A metal wire was selected as a waveguide, and pencil breaks and electronic pulses were used as artificial AE signals. The results indicate that the use of the surface waveguide can significantly increase the AE monitoring range. Also, a high transmission efficiency was experimentally determined for the epoxy joints developed to attach the surface waveguide to the FRP pipe. The proposed surface waveguide appears to be a promising technique for AE monitoring on existing FRP pressure vessels and storage tanks.

Remote field eddy current defect interactions: effects on the external field

Abstract: Finite element calculations for the remote field eddy current (RFEC) through-wall inspection technique show that metal loss on either the inside or the outside affect the exterior field as well as the interior transmitted remote field. In ferromagnetic tubes, wall thinning decreases attenuation and phase lag, as found experimentally. In the nonferromagnetic pipe modeled, attenuation decreased but wall thinning increase the phase lag. The exterior field changes are attributed to inner surface reflections of the field diffusing from the exterior to the interior of the pipe.

Return on investment from NDE and maintenance expenses -- A case history

Abstract: NDE can generally be shown as an important component of power station maintenance programs. Operational performance records, along with maintenance and inspection costs of three fossil units,have been examined to determine the effectiveness of the inspection program employed by Arizona Public Service (APS) at the Four Corners Power Station. Data from the last 18 years revealed that the effective use of NDE to support maintenance activities has been a significant contributor to the excellent reliability of three coal fired units, all now over 30 years old. The plant burns coal that is extremely high in ash. Consequently, the maintenance overhauls are very aggressive. Both maintenance and inspection costs are comparatively high when viewed independently. However, examined in the context of the availability factors and the cost per megawatt hour (MWH), they are brought back into proper perspective. Another positive benefit of the APS programs has been to improve boiler availability, and consequently net generation, over the past 10 years in spite of the advancing age of the units. This article gives an overview of the APS program and how NDE is used in planning for boiler maintenance activities and during annual overhauls. It illustrates the improved unit availability factors andmore » operating cost reductions that result from use of systematic NDE in support of aggressive plant maintenance.« less

Monitoring processing stages of carbon-carbon composites using vibration based nondestructive evaluation

Abstract: In the processing of carbon-carbon (C/C) composites, development of delaminations and excess damage during carbonization is of serious concern. Continuation of the process in spite of such occurrences will result in a material with poor mechanical properties. This is because, even on further impregnation of the composite by the matrix precursor, the material remains weak at the sites at which delaminations occurred. There is hence a definite need to examine materials at each stage of processing in order to gain knowledge about the overall state of the material. Of the several available nondestructive evaluation (NDE) techniques, in this work vibration-based NDE is chosen due to its sensitivity to changes in the global material condition. In this paper, the focus is on measurement of vibration-based NDE parameters, namely resonance frequency and damping ratio values at various processing stages of C/C composites. Microstructural investigations pertaining to several stages of processing of these composites are presented. The results indicate that vibration parameters are extremely sensitive to changes in microstructure and mechanical property variations in C/C composite materials.

Photothermal radiometry apparatus using pseudorandom excitation for nondestructive evaluation of layered materials

Abstract: The characterization of thermal system under unsteady regime is mostly carried out under pulsed or sinusoidal excitation, as they are easier to produce, analyze, and model. The new means of study offered by optics and computers now allow us to operate and exploit other modes of thermal analysis of systems. The equipment presented uses random type excitation and is to be applied to nondestructive testing. Yet, the scope of this method can be enhanced. In fact, it can be tied to different means of thermal stimulation as well as to various types of sensors. It can be more generally used for the analysis of different thermal systems under unsteady regime.

Repair grinding ― An alternative to ultrasonic rejection for oilfield tubes

Abstract: The price of scrap steel rose from $95 to $140/ton in 1993, making tubular production costs higher and making internal repair grinding of oil field tubes with minor internal surface breaking imperfections an extremely cost effective method for placing potentially reject tubulars back into service. Such imperfections often occur and are presently sized by the shear wave ultrasonic amplitude method. This article is written to illustrate how effective repair grinding can be in producing prime material within the API specifications 5CT, specification for Casing and Tubing; 5D, Specification for Drill Pipe; and 5L, Line Pipe. Ninety percent of high grade and 50--60% of midgrade materials can be saved by this method.

Vision and the detection of magnetic particle indications

Abstract: Magnetic particle testing (MT) can be divided into two separate and interrelated processes, both of which are essential in the course of an effective examination. The first of these is the formation of the indication, whereby a ferromagnetic object is magnetized in a manner that induces a field of given magnitude and direction. A crack or similar surface defect perpendicular to that magnetic field causes the formation of a magnetic flux leakage field. The second process is the visualization of the field whereby fine iron oxide particles, usually pigmented, are magnetically drawn to that leakage field. As industry calls for higher productivity -- meaning greater inspection volume and accuracy in less time and with less labor -- every avenue of the inspection processes should be reviewed. This article addresses the subject of what makes the MT indications visible. It views the vision system used for detecting particle indications as having three primary components: (1) the source, the fixture emitting visible light or ultraviolet (UV) radiation; (2) the object, the particle indication; and (3) the sensor, being either the human eye or a camera.